Cooled current leads are now being used. U.S. Pat. No. 3,946,142 issued Mar. 23, 1976 to M. Kellow et al entitled "Cooling of Power Terminals Utilizing an Open Cycle Cooling System" discloses an apparatus for cooling underground power cables. This patent is incorporated herein by reference. In the Kellow et al patent, the apparatus comprises a chamber adapted to enclose a length of underground power cables. Liquid and vapor are provided to cool the cables. In various "stations", a cooling fluid is pumped around a portin of the outside of the cable as well as through that portion of the cable contained within the "station" with water being the cooling fluid used.
U.S. Pat. No. 3,257,873 issued Sept. 8, 1970 to H. Brechna et al entitled "Composite Superconducting Cable in a Porous Matrix" discloses a composite superconducting cable supported in a conduit by means of a porous matrix provided with a main channel for circulation of a coolant through the cable. This patent is hereby incorporated by reference. In the Brechna et al patent, the porous matrix both supports the superconducting wire and at the same time permits penetration of coolant directly to the wire. A cryogen in a super critical state is circulated through the cable, moves through the porous matrix and penetrates directly through the superconducting wires and removes heat. The cooling fluid washes around the outside of the solid superconducting wire.
The above two patents are inadequate for providing solutions to the problems from which the invention described herein arose. The specific problems of the prior art were: (1) how to provide electric current to a superconducting electromagnet through a current lead which would not transmit heat from the outside atmosphere to the superconducting electromagnet, and (2) how to provide a current lead which had means for dissipating Joule heating occurring within the current lead as the electric current passed through the lead.
The phenomenon of superconductivity i.e., zero electrical resistance, is produced commonly by cooling a superconducting alloy such as niobium-tin or niobium titanium, below a critical temperature. For the most common superconducting alloys, the critical temperature is 18.degree. K. or less and may be achieved with a liquid cryogen coolant, such as liquid helim which has a temperature of 4.2.degree. K. under standard atmospheric conditions. However, the actual operating temperature of a particular superconductor is influenced by several factors, including the amount of current flowing in the conductor, the magnitude and rate of change of any magnetic fields to which the conductor is subjected. In particular, current conduction above a critical level causes a temperature rise in the conductor due in part to Joule heating; this results in a partial loss of zero resistance, thereby producing localized heating.
Since the normal resistance of superconductors is realatively high compared to copper or aluminum at cryogenic temperatures, any rise of the temperature of the conductor above the critical level causes local heating which may result in heating of adjacent areas and eventually the entire conductor. When this happens, the superconductor reverts from zero resistivity to its normal resistance, causing Joule heating in the conductor. Due to the high "normal resistance" (i.e. the inherent resistance of the alloy at non-superconducting temperatures) of superconducting alloys, the temperature may become so high as to destroy the conductor.
Vapor cooled current leads are presently being used to supply electric current to superconducting electromagnets. In one apparatus now in use, enclosed in a shell is a bundle of parallel electrical conductors which are hollow in the center throughout their length. The conductors of this apparatus are made of copper alone and bundled together inside the shell. The inside ends ("cold ends") of these conductors are inserted into and joined to the superconducting electromagnet, and cooled by evaporating a liquid cryogenic coolant into a vapor which passes through the hollow centers of the conductors. The current leads are electrically connected to an external current force. As current moves from the current source through the conductors into a superconducting electromagnet, heat is generated by Joule heating which is of sufficient magnitude to melt the tubes if the tubes are not cooled. Cooling is provided by flowing the vapor coolant originating from inside the superconducting magnet into and through the conductors, the coolant absorbing heat from the tubes and venting through ports provided at the outside end of the shell toward the outside end ("warm end"), and also through the hollow centers of the tubes.
If there is a loss of coolant flowing through the tubes carrying the current, in approximately five minutes the tubes will experience thermal failure, thereby stopping the flow of current through the superconducting electromagnet. Because of the nature of operation of superconducting electromagnets, such a sudden loss of current would severly damage if not destroy superconducting electromagnets. The problem became one of extending the period of time it took the current leads to heat up enough to experience thermal failure, thereby permitting a greater period of time over which the current can be gradually reduced to "turn off" the superconducting electromagnet without a catastrophic failure. Up until the time of the invention disclosed herein, there has been no sufficient way to deal with this problem.